The modification of materials with groups derived from polyethylene glycol (PEG) is known as PEGylation. PEGylation is now widely used for the modification of bioactive molecules, such as proteins, peptides, antibody fragments, oligonucleotides, and the like for, use as drugs.
Although bioactive molecules such as protein and peptide drugs hold great promise as therapeutic agents, many are degraded by proteolytic enzymes, can be rapidly cleared by the kidneys, generate neutralizing antibodies, have a short shelf life, have low solubility, and/or have a short circulating half-life. PEGylation of these materials can overcome these and other shortcomings. The ability of PEGylation to decrease clearance typically is not a function of the number of PEG groups attached to the protein, but is related to the overall molecular weight of the altered proteins. By increasing the molecular mass of proteins and peptides and shielding them from proteolytic enzymes, PEGylation improves pharmacokinetics. Among the other advantages of PEGylation are: increased water solubility, increased bioavailability, increased blood circulation, decreased protein aggregation, decreased immunogenicity, reduced toxicity, and decreased frequency of administration.
Branched polymers can provide a single non-linear polymer molecule with a high overall molecular weight. Branched or star-shaped polymers comprising a plurality of polymer arms attached to a central core and having a single reactive group for conjugation to a biologically active molecule have been described, for example, in U.S. Pat. Nos. 5,643,575, and 5,932,462, the disclosures of which are both incorporated herein by reference. Although these branched polymers are useful for attaching a high molecular weight polymer to a molecule at a single attachment site without using an extremely long polymer chain, the methods for forming the branched PEG molecules are difficult and require extensive purification of the PEG polymers prior to attachment to the core molecule as well as purification/removal of partially PEGylated polymer intermediates following attachment.
WO 2006/003352, the disclosure of which is incorporated herein by reference, describes the use of atom transfer polymerization (ATRP) and reversible addition fragmentation transfer (RAFT) in the preparation of comb polymers from monomers that contain alkoxy polyethers. The disclosure describes many shortcomings of the prior art and benefits of using a controlled polymerization process, however, ATRP polymerizations also have several drawbacks including, but not limited to, slow polymerization kinetics, residual metallic byproducts, scale-up difficulties, and limited polymer composition and molecular weight ranges. The metallic by-products can be detrimental in biological systems and require removal, which is difficult and requires laborious procedures. U.S. Pat. No. 6,610,802, for example, describes these byproducts and discloses the disadvantage of ATRP processes. Furthermore, the large amount of metallic control agent required can cause discoloration as well as corrosion issues in some reactors. Sensitivity to oxygen and to certain functional groups, such as acids, is an additional limitation encountered with this technique as it leads to poor control and impurities.
RAFT uses dithio esters of carbamates, xanthates, and trithiocarbonates, such as dibenzyl trithiocaronate (DBTTC) as radical control agents. However, for the RAFT process to function effectively, the RAFT agent must be carefully chosen based on the type of monomer used and the polymerization rate. The RAFT technique also has limitations in obtaining well-defined functionalization as not all polymer chains will have the desired end-functionalization due to the need for external polymerization sources. Odor and discoloration due to the sulfur-based control agents are also drawbacks of this technique. In addition, the ATRP and RAFT techniques both suffer from by-product contamination and product purification problems.
Thus, a need exists for a controlled method for preparing tailored polymers containing the PEG group that provides flexibility in their design but does not have these disadvantages.
The delivery of functional agents, which are defined herein as; molecules, bioactive molecules, ingredients, or compositions such as flavors, fragrances, pharmaceuticals or pesticides, agrochemicals such as herbicides, fungicides, or pesticides, dyes, and many others are an important aspect for nearly all applied sciences. Without the stabilization of a concentrated, easily transportable and processable form of the functional agent, delivery becomes unreliable and the agent will only rarely exhibit its beneficial properties at the predetermined place and time. Effective encapsulation is required in a wide range of applications in order to protect sensitive additives from degradation and to control their release, which will optimize their performance according to the required application.
There are many different encapsulation technologies apart from PEGylation available. One such available technique is to use amphiphilic block copolymers (polymers having hydrophilic and hydrophobic block segments). Amphiphilic block copolymers are well known to form micelles in aqueous solution making them suitable for encapsulation or solubilization of hydrophobic or water insoluble agents. Encapsulation technologies and specifically the use of amphiphilic block copolymers are described for example in U.S. Pat. Nos. 5,939,453, 6,638,994, US Patent Publications 2007/0160561, US2004/0010060, and 2005/0180922 the disclosures of which are incorporated herein by reference. These references describe amphiphilic polymers prepared by the aforementioned ATRP or RAFT methods and are limited by the drawbacks associated with these techniques (previously described above). Also described is a class of PEO based amphiphilic block copolymers made by living anionic polymerization techniques.
Living anionic polymerization suffers from several drawbacks, such as, poor copolymerization between polar and non-polar comonomers and the inability to use monomers that can be easily deprotonated. Therefore functional monomers cannot be directly incorporated and the copolymerization of monomer mixtures can be problematic and/or non-viable. This reduces the ability to tailor properties such as solubility, reactivity, and Tg. Furthermore, this process can be expensive, difficult or impractical to carry out on an industrial scale as bulk or emulsion techniques cannot be used, extremely pure reagents are necessary (even trace amounts of protic material inhibits polymerization), and an inert atmosphere is requisite.
These references either use techniques that are not amenable to tailoring specific properties through copolymerization, gradient copolymers, functionalization or fail to teach the significance of tailoring block composition or allowing for the formation of gradient compositions to control both agent solubility and agent release.